[PATCH] Add support for http_proxy and https_proxy environment variables

The attached patch implements support for the http_proxy and https_proxy
environment variables. This allows subversion to automatically make use
of system-wide proxy configuration, rather than requiring
subversion-specific proxy configuration; environment variables also make
it easier to have network-specific configuration on machines that use
more than one network. Subversion-specific configuration (in
~/.subversion/servers) will override the environment.

Written after observing some ChromeOS build system documentation that
showed how to configure proxy environment variables, and then listed
subversion as the one special case requiring its own configuration.

- Josh Triplett